This invention relates to a ceramic-metal composite article and fabrication method therefor. In particular this invention relates to a design geometry and material system for a brazed joint between the ceramic and metal components of a ceramic-metal composite article for load bearing and high temperature applications.
Various ceramic materials have been proposed as replacements for metal component parts in high temperature environments, including the high temperature corrosive environments found in high fuel efficiency, high temperature internal combustion and gas turbine engines. Such replacements are commonly referred to as "ceramic heat engine components." Ceramics proposed as suitable for such use include, e.g., silicon nitride, silicon carbide, zirconia, and alumina, particularly as reinforced composite materials.
However, for economic, production, and other reasons, the ceramic normally is used for only those portions of the engine components actually exposed to the high temperature environment, while less heat resistant metal may be used for the component portions not so exposed. The result is a composite body including both ceramic and metal portions, requiring a strong, reliable joint between the two materials. For simplicity, these composite bodies are also referred to herein and in the appended claims as ceramic heat engine components. Typically, temperatures in the above-described engines rise from room temperature to about 1200.degree. C. In such an engine, the ceramic-metal joint of, for example, a ceramic rotor will be exposed to temperatures of up to about 600.degree.-650.degree. C. A ceramic-to-metal joint for this application should withstand the high temperature corrosive environment of the engine, as well as the various stresses to which it is subjected in use.
A major problem in joining ceramics to metals is the thermal mismatch between the ceramic and metal materials. Thermal mismatch causes significant residual stress in the ceramic, possibly leading to catastrophic failure during production or in service.
Ceramic-metal joints have used various joining methods, e.g. glass frit, diffusion bonding, brazing, and mechanical shrink fit, and various joint geometries, e.g. butt, conical, and cylindrical joint geometries. The cylindrical joint has been most often associated with a mechanical shrink fit (press fit) technique. To effect a shrink fit, a relatively simple, straight cylindrical bore into the metal member is required. However, a shrink fit joint not only requires strict dimensional tolerances between the metal and ceramic members, but also limits the use temperature of the joint to a temperature far below the processing temperature of, usually, about 500.degree. C.
Brazing has also been proposed as another potential joining technique for various ceramic-metal joints including the cylindrical joint. Brazing techniques potentially can ease the close machining tolerances required for shrink fit joint components, as well as provide superior performance in high temperature components. However, when brazing is used as a joining method, the straight, cylindrical bore normally used for a shrink fit joint causes another set of problems, e.g. gas entrapment, subsequent misalignment, and nonuniform bond area. Further, few joint geometries have been successfully designed specifically for the brazing approach to composite high temperature structural parts.
It would be of great benefit to the development of high fuel efficiency, high temperature ceramic engines, as well as other high performance technologies, if processing methods, joint designs and joint material systems could be developed to consistently produce reliable joints in ceramic-metal components, and if machining tolerances for ceramic and metal parts could be made less critical than those required for the shrink fit approach. Typically, such joints should fulfill performance requirements both at room temperature and at high temperatures.